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of 3 300 m (11 000 ft) and datum point to threshold distance of 230 m (760 ft)), with a 3-degree elevation angle. Within
3.7 km (2 NM) of the MLS approach reference datum, the portion of the CMN budget reserved for computational error is
1.1 m (3.5 ft) laterally and 0.6 m (2.0 ft) vertically.
Note.— All errors represent 95 percentile errors.
13.3 Siting and accuracy considerations
13.3.1 Theoretical and operational analysis has shown that several factors will impact the amount of azimuth antenna
lateral offset that can be permitted and still obtain lateral and vertical position accuracy identified in 13.2.
13.3.2 Distance between azimuth and elevation antennas
13.3.2.1 For a given azimuth antenna offset, a short azimuth to elevation distance results in relatively large azimuth
angles at positions near the approach reference datum. As a result, the error contribution from the DME is large, and the
lateral accuracy may degrade unacceptably. At a runway where a large azimuth antenna offset and a short azimuth to
elevation distance exist, use of DME/P rather than DME/N may be required to achieve the required lateral accuracy.
13.3.3 Azimuth accuracy
13.3.3.1 The azimuth antenna offset limits presented in 13.5 are based on the ±6 m (20 ft) azimuth path following error
accuracy specification (see Chapter 3, 3.11.4.9.4). The recommended ±4 m (13.5 ft) azimuth accuracy specification would
permit larger azimuth antenna offsets and still obtain required computed position accuracy at DH. Azimuth angle accuracy is
assumed to degrade in accordance with Chapter 3, 3.11.4.9.
13.3.4 DME accuracy
13.3.4.1 Smaller errors in position determination result when DME/P equipment is used and the final approach
segment is contained within 9.3 km (5 NM) of the MLS approach reference datum. There are two DME/P final approach
mode accuracy standards in this region. Resulting azimuth antenna offset values when using DME/P as presented in 13.5, are
based on final approach mode Standard 1 accuracy. Larger azimuth antenna offset values may be permissible if DME/P
equipment meeting final approach mode Standard 2 accuracy is used. DME/P final approach mode Standard 1 ranging
accuracy is assumed to degrade in accordance with Chapter 3, 3.5.3.1.3.4 and Table B. DME/N is assumed to degrade in
accordance with Chapter 3, 3.5.3.1.3.2.
13.3.5 Use of elevation information in the lateral position computation
13.3.5.1 Generally, lateral position computation that excludes elevation information will be sufficient for computed
centre line approaches to the primary runway. If elevation information is not used in lateral computation, the lateral error
Annex 10 — Aeronautical Communications Volume I
increases. This error increases with azimuth angle, height and decreasing range. Permissible azimuth antenna offsets
presented in 13.5 are reduced if elevation information is not used in the lateral computation. Elevation angle accuracy is
assumed to degrade in accordance with Chapter 3, 3.11.4.9.
13.4 Equipment considerations
13.4.1 Performance of airborne sensors, MLS ground equipment and MLS/RNAV avionics implementation influence
the range of application of computed centre line approaches. Information presented in 13.5 is based on the following
equipment considerations.
13.4.2 Airborne sensors
13.4.2.1 It is assumed the receiver will decode all auxiliary data words required for MLS computed centre line
approaches unless the information contained in the data words is available from other avionics sources with the same
accuracy and integrity as required for auxiliary data. Digital MLS angle data and range data are needed for computing lateral
and vertical position. Angle data quantization is 0.01 degrees. Range quantization is 2.0 m (0.001 NM).
13.4.3 RNAV computations
13.4.3.1 No assumption is made about where the RNAV position computations are made. A portion of the computed
centre line approach error budget has been reserved for computation error. This permits flexible algorithm implementation.
13.4.4 Permissible azimuth antenna offset calculation techniques
13.4.4.1 RTCA (RTCA/DO-198, Appendix D) has identified several different position determination algorithms.
Different algorithms can handle different ground equipment configurations. The algorithm designed to handle any ground
equipment geometry is the RTCA case 12 algorithm. Permissible antenna offset values were obtained using Monte Carlo
simulation techniques. The results were also obtained using a direct analytical method. The analytical method uses geometric
transformations of the maximum MLS angle and range errors to determine system performance. The Monte Carlo
technique through the emulation of an MLS/RNAV system is a statistical method used to determine system performance.
　
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